function [Z] = second_f(x, y, k, l, a)
Z = zeros(length(x), length(y));
for i = 1:length(x)
    for j = 1:length(y)
        for s = 1:length(k)
            Z(i,j) = a(s)*sin(k(s)*x(i))*sin(l(s)*y(j))+Z(i,j);
        end
    end
end